4
votes
1answer
274 views

Why is H(message||secret_key) not vulnerable to length-extension attack?

Given a Merkle-Damgård hash function $H$, I know that an attacker can forge a message protected by a MAC computed as $H(\textrm{secret_key}||\textrm{message})$. Why can't he perform the same ...
17
votes
3answers
545 views

Purpose of outer key in HMAC

From what I know, the HMAC constructions has two strength: It's resistant to length extensions Since the key is consumed before the message, the attacker does not know the initial state, preventing ...
7
votes
1answer
757 views

In which situations is a length-extension attack a problem?

A lot of hash functions, including the SHA-2 family(but not the SHA-3 candidates and SHA256d) are vulnerable to length extension attacks. But when is this property a problem? I guess certain naive ...